In a medium soup pot, heat olive oil and sauté onion, carrots, celery, and mushrooms until onions are just tender, 3-5 minutes. Stir in lentils. Add 7 cups broth, bay leaf, rosemary, oregano, basil, red pepper flakes, and garlic. Bring to a boil, reduce heat, and cook on medium, covered, for about 20 minutes, until lentils are tender.

Remove bay leaf. Purée half of the mixture and return to pot. If desired, add remaining 1 cup stock to reach desired consistency. Stir in baby spinach leaves and cook for 1 minute. Season with salt and pepper. Top each serving with a spoonful of sherry or sherry vinegar, if desired.